Adverse Prognostic Features in Chronic Lymphocytic Leukemia
Adverse Prognostic Features in Chronic Lymphocytic Leukemia
ABSTRACT: Patients with chronic lymphocytic leukemia (CLL) can have variable courses, from indolent disease to rapid progression with limited response to treatment. The Rai and Binet staging systems were the first to classify patients into prognostic categories. Newer prognostic markers that correlate with shorter time to progression and time to treatment include elevated serum β2M, TK, ZAP-70, and CD38, as well as unmutated IgVH. Abnormal cytogenetics are found in the majority of patients with CLL. Del(17p), as well as mutations of TP53, is associated with an aggressive clinical course and short overall survival. Nearly one-fifth of patients have del(11q) and have a significantly shorter median progression-free survival; mutations in the ATM gene, located on 11q, may also have adverse prognostic implications. Intermediate-risk cytogenetic findings include trisomy 12 and del(6q). Patients with del(17p) should be evaluated for novel agents and/or allogeneic stem cell transplantation in first remission. Patients with del(11q) require treatment with an alkylating agent in addition to nucleoside analogs and rituximab, and patients with trisomy 12q may express higher levels of CD20, thereby making the malignant cells more susceptible to biologic agents that target CD20. Despite advances in stratifying patients and improved chemoimmunotherapeutic regimens, additional research in prognostication and treatment is needed.
Chronic lymphocytic leukemia (CLL) is a chronic lymphoproliferative disorder characterized by monoclonal lymphocytosis. Between 2003 and 2008, the age-adjusted incidence rate was 4.2 per 100,000 persons per year, with an annual age-adjusted death rate of 1.5 per 100,000. Although CLL has historically been considered an indolent, slow-growing disease, it progresses rapidly in some patients and they die within 2 to 3 years of diagnosis. Given the heterogeneous nature of the disease course, accurate predictors of prognosis are valuable for the clinical care and counseling of patients. The majority of patients are asymptomatic at the time of diagnosis, and use of a variety of prognostic measures can predict rapid disease progression and responses to treatment (Table). Although models for determining prognosis for patients with CLL have existed for over 35 years, these relied on simple clinical characteristics such as physical examination findings and blood counts. The complex biologic pathways in CLL are gradually being revealed, and the implications of abnormalities in these pathways, both for treatment and prognosis, are adding to our understanding of this heterogeneous disease.
Early Prognostic Classification Models
The first prognostic model was proposed in 1975 by Rai et al. The Rai staging system uses the presence or absence of lymphadenopathy, organomegaly, anemia, and thrombocytopenia to classify patients into one of five stages, stage 0 through stage IV. Median survival for these stages has been found to range from 19 months for stages III and IV to > 150 months for stage 0..
The Binet staging system classifies patients into three categories based on the number of lymphoid tissues involved—including cervical, axillary, and inguinal lymph nodes, as well as liver and spleen—and the presence of anemia and/or thrombocytopenia. Group A patients have no thrombocytopenia or anemia and fewer than three lymphoid tissues involved, group B patients have no thrombocytopenia or anemia but three or more lymphoid tissues involved, and Group C patients have anemia and/or thrombocytopenia. Published median survival rates in these patients ranged from 2 years for patients in stage C to a survival similar to that of age-matched controls for patients in stage A. However, these two staging systems examine patients' clinical status at a single point in time and cannot differentiate between indolent disease and disease that will subsequently become aggressive.
The lymphocyte doubling time, defined as the length of time it takes the absolute lymphocyte count to double from diagnosis, has also been found to have prognostic value in patients with CLL. In one study, those patients whose lymphocyte counts doubled in less than 12 months had a significantly shorter median survival (36 months), whereas those with a longer lymphocyte doubling time experienced longer overall survival (OS) (median survival was not reached). Patterns seen on bone marrow examination also have prognostic value. A diffuse pattern of involvement, including destruction of normal bone marrow architecture and replacement of hematopoietic cells and fat cells with lymphocytes, was associated with decreased OS. Lymphocyte doubling time requires serial blood measurements over time; in addition, the absolute lymphocyte count can vary as a result of other events unrelated to the malignancy. Bone marrow examination is invasive and requires a certain degree of subjectivity, and a diffuse pattern is more often seen in patients who present with already advanced disease. Therefore, these two prognostic factors have significant limitations.
Newer Prognostic Factors
Recent research has been directed toward biomarkers and genetic abnormalities that are independent of clinical stage. These abnormalities include IgVH mutation status as well as ZAP-70 and CD38 expression.
Serum levels of beta 2 microglobulin (β2M), a component of the HLA class I complex on nucleated cells, are elevated in a variety of hematologic malignancies. Measurement of β2M in serum is easy, fast, inexpensive, and highly reproducible. Serum β2M levels correlate with progression-free survival (PFS).[6-8] Patients with β2M levels less than 3.5 mg/L have a substantially longer time to progression than those with levels greater than 3.5 mg/L, and elevated β2M levels are associated with lower rates of complete remission and shorter failure-free and overall survival.[9, 10] Adjusting the β2M level for creatinine clearance may improve its predictive ability.
Thymidine kinase (TK) is an intracellular protein involved in a salvage pathway for DNA synthesis, so increased expression of TK may reflect increased lymphocyte proliferation. Elevated levels of TK are predictive of disease progression, although there is some disagreement on the optimal value to use to stratify patients. In a small study of 58 patients, TK > 15 U/L was the single best predictor of germ-line IgVH status, with a predictive value of 86%. Using the value of 8.5 U/L as a cutoff, Matthews et al found that TK level correlated with rapid lymphocyte doubling time, time to progression, ZAP-70 expression, CD38 expression, and poor prognostic cytogenetic findings. TK has been shown to have treatment implications as well; levels greater than 10 U/L are associated with a lower rate of response to treatment with fludarabine.
Mutations in the immunoglobulin heavy chain variable gene (IgVH), defined as a greater than 2% difference from the germ-line nucleotide sequence, are seen in approximately 50% of patients and are associated with longer survival.[15,16] In one study, patients with IgVH mutations had a median survival of 293 months, whereas those with the germ-line DNA sequence had a median survival of 95 months. Once patients have been treated, mutation status does not affect the chance of complete remission (CR) in those receiving fludarabine, cyclophosphamide, and rituximab (Rituxan) (FCR); however, patients with unmutated IgVH had a shorter remission duration (47% vs 82% at 6 years). Mutation status remains a prognostic factor even after autologous stem cell transplantation. However, determination of the presence of somatic mutations in the IgVH region requires DNA sequencing, which is not readily available, so the test is difficult to perform in many centers.
Zeta chain associated protein 70 (ZAP-70) is a tyrosine kinase involved in cellular signaling of T cells; it is abnormally expressed in the malignant B cells of some patients with CLL. It is measured by flow cytometry, and its expression has been shown to correlate with the presence of unmutated IgVH genes. ZAP-70 overexpression has been observed in 36% to 57% of study populations; [21-24] however, these trials have used different cutoff values for abnormal expression. It is now generally accepted that expression of ZAP-70 in the cytoplasm of more than 20% of cells represents abnormally high expression.[23,24] ZAP-70 positivity portends a poor prognosis and a shorter time to treatment compared with ZAP-70 negativity; times to treatment were reported as 34 vs 130 months in one study and 2.6 vs 8.4 years in another. The level of ZAP-70 expression was not found to change significantly over the duration of the study. ZAP-70 is highly expressed by T cells, which may cloud interpretation of results; in addition, unlike β2M, it is an intracellular protein, which makes flow cytometry more difficult. Measurement of ZAP-70 by flow cytometry is also highly variable and can depend on the conditions of the sample, its handling and preparation, and the methodology and materials used.[23,25]
CD38, a cell surface glycoprotein normally expressed in a wide variety of hematopoietic cells, is easily measured by flow cytometry in the laboratory. Like ZAP-70, CD38 positivity is associated with faster progression, resistance to treatment, and decreased OS; its expression also correlates with IgVH mutation status.[27,28] In one study, however, there was a 28% discordance rate between CD38 expression and IgVH mutation status, indicating that they are independent prognostic indicators . Studies have reported that 29% to 47% of patients demonstrate CD38 positivity (CD38+), which in most cases (but not all) is defined as expression in 30% or more of cells as determined by flow cytometry.[16,23,24,26-28] Rassenti et al reported a clinically significant difference of 1.7 years between diagnosis and first treatment for CD38+ vs CD38- patients (4.0 vs 5.7 years). Reported median survival for CD38+ patients is shorter (range, 30 months to 10 years) compared with that in CD38- patients (range, 100 months to longer than the study period).[16,24,26-28]. CD38 levels may change throughout the course of the disease.